RESUMO
Spontaneous secretion of GH decreases with aging. To investigate whether fasting increases pulsatile GH secretion in older as it does in younger subjects, we studied six subjects (four postmenopausal women and two men, aged 55-81 yr; body mass indexes, 22-24 kg/ m2). Blood was obtained every 5 min for 24 h on a control (fed) day and on the second day of a fast. Serum GH concentrations, measured by an immunoradiometric assay, were analyzed with a multiple parameter deconvolution method to stimultaneously resolve endogenous GH secretory and clearance rates. Two days of fasting induced a 4-fold increase in the 24-h GH production rate (38 +/- 25 vs. 166 +/- 42 micrograms/L distribution volume; P = 0.003) and a 2-fold increase in the amount of GH secreted per pulse (2.4 +/- 1.4 vs. 5.5 +/- 1.2 micrograms/L distribution volume; P = 0.02). The latter was a result of increased secretory burst amplitudes with unchanged secretory burst durations. The number of detectable GH secretory bursts per 24 h was also increased by fasting (13 +/- 1.4 vs. 30 +/- 1.1; P = 0.0004); the GH pulse frequency may have been underestimated in the fed state, as 33 +/- 4.9% of the samples had undetectable ( < 0.2 microgram/L) serum GH concentrations compared to 5.2 +/- 2.6% of the samples on the fasting day (P = 0.004). The t1/2 of endogenous GH was not significantly altered by fasting. The fold increase in GH secretion with fasting was similar to that previously observed in young men, although absolute levels of GH secretion were approximately 50% lower in both fed and fasted conditions. Fasting decreased the proportion of sleep time spent in rapid eye movement sleep (4.7 +/- 1.3 vs. 15 +/- 2.1%; P = 0.005), but did not significantly increase slow wave (stages 3 and 4) sleep. In both fed and fasted conditions, mean GH secretion rates were similar during daytime wakefulness, nocturnal wakefulness, rapid eye movement sleep, and stages 1, 2, and 3 of sleep. We conclude that hyposomatotropism associated with aging is partially reversed by fasting, and the enhancement of GH secretion by fasting is not related to changes in slow wave sleep. These data indicate that GH secretion in older persons can be enhanced by physiological interventions.